Pb isotopic tracers of the Cretaceous‐Tertiary extinction event

Abstract

The global excess of Ir in sediments at the Cretaceous‐Tertiary (K‐T) boundary has been attributed to either a meteorite impact or enhanced volcanism (e.g. Deccan Traps). The isotopic composition of Pb associated with this Ir provides a test of these hypotheses because meteoritic Pb is isotopically quite distinct from Pb of volcanic or continental origin. Pb abundances and isotopic compositions of pelagic oozes from DSDP Site 577A (Shatsky Rise, NW Pacific) measured in this study show little change over the deposition interval of Ir‐rich sediment (∼10 cm). However, a Pb enriched layer with a more radiogenic isotopic composition is found in the basal ∼0.5 cm of the Tertiary. These observations appear to preclude the Ir excess in these sediments originating as impact fallout from a stony meteorite although an iron meteorite impactor cannot be excluded. Any Pb flux accompanying Ir in volcanogenic aerosols from the Deccan Traps should be considerable. However, the boundary Pb ‧spike‧ does not isotopically resemble known Deccan Trap lavas. There is isotopic overlap with coal ash although the lack of correlation between soot and Pb abundance precludes any simple relationship between the two. Overall, any wind pattern changes following the K‐T ‧event‧ were apparently short‐lived as neither airborne sources of Pb nor their magnitude to the then central Pacific were affected while the Ir enriched sediments were being laid down.